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Demographic consequences of early primiparity: importance of food availability and individual
heterogeneity
Demographic consequences of early primiparity
Age at which females first reproduce
Early maturation should be selected for because:
period during which animals risk dying before reproducing
•early breeders compound their genetic interest before those delaying their first reproduction
effect on fitness of individuals
effect on population growth rate
Demographic consequences of early primiparity
But lots of species differ their first reproduction after physiological maturity
=> Reproducing early in life may be costly
Trade-offs:
•early reproduction - growth
•early reproduction - survival
•early reproduction - future reproduction
Demographic consequences of early primiparity
Importance of the food availability in the expression of reproductive costs
Fitness costs of early reproduction are important when food availability is low and negligible when
food availability is high
Demographic consequences of early primiparity
Few studies about the demographic consequences of early primiparity
Some examples on ungulates, rare on small mammals
The Kluane Red squirrels population
Red squirrels at Kluane:
•long-term dataset, data on reproduction, age of individuals etc, etc
•food availability index
•<40% of females breed at 1 year of age (age of physiological maturity)
First hypothesis:
females who bred at 1 year of age after a cone failure have lower fitness components than females delaying their first
reproduction after a cone failure. Opposite difference after a mast year.
The Kluane Red squirrels population
Food availability
Fit
ne
ss
Early primiparityDelayed primiparity
First hypothesis and results
Hypothesis
Cone production the year of birth
<10 <200 >200Lif
etim
e re
pro
du
ctiv
e su
cces
s
0
1
2
3
4
5
6
Results
First hypothesis and results
Cone production the year of birth
<10 <200 >200
Lo
ng
evit
y
200
400
600
800
1000
1200
1400
1600
Cone production the year of birth
<10 <200 >200
Ave
rag
e b
reed
ing
su
cces
s
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Cone production the year of birth
<10 <200 >200
Nu
mb
er o
f b
reed
ing
eve
nts
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Early primiparity
Delayed primiparity
First hypothesis and results
Results opposite to the prediction: higher fitness for females who bred at 1 year of age after a cone failure than females
who delayed their first reproduction after a cone failure
Alternative (non exclusive) hypothesis
Only females of high phenotypic quality reproduced when low food availability (i.e. after a cone failure).
(previous examples on birds)
New Hypothesis
Cone production the year of birth
<10 <200 >200Lif
etim
e re
pro
du
ctiv
e su
cces
s
0
1
2
3
4
5
6
Results
Alternative (non exclusive) hypothesis
Food availability
Fit
nes
s
Early primiparityDelayed primiparity
Alternative (non exclusive) hypothesis
Prediction 1: higher survival of females breeding at 1 year of age after a cone failure than females breeding at 1 year of age after a mast year
Age
0 1 2 3 4 5 6 7 8 9
Su
rviv
al r
ate
s
0.0
0.2
0.4
0.6
0.8
1.0
Early primiparity after a cone failure
Early primiparity after a mast year
Alternative (non exclusive) hypothesis
Prediction 2: for the first reproductive event, higher mass and/or larger litter size and/or higher mass of juvenile produced for females breeding at 1 year of age after a cone failure than females breeding at 1 year of age after a mast year
Cone production the year of birth
<10 <200 >200
Ma
ss
aft
er
the
pa
rtu
riti
on
235
240
245
250
255
260
265
270
275
Cone production the year of birth
<10 <200 >200
Av
g m
as
s o
f th
e w
ea
ne
d j
uv
en
ile
s
145
150
155
160
165
170
175
Alternative (non exclusive) hypothesis
Prediction 3: lower survival for females breeding at 1 year of age after a mast year than females who delayed their first reproduction after a mast year
Age
0 1 2 3 4 5 6 7 8 9
Su
rviv
al r
ates
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Early primiparity
Delayed primiparity
Conclusion
Our alternative hypothesis is supported by our data
•After a cone failure, only females of high quality breed at 1 year old and they achieve a better fitness (despite the cost of early
reproduction)
•After a mast year, no confounding effect of the individual quality. The costs of early reproduction (on survival and breeding
success) can be detected despite the high energy available
Conclusion
Long-term cohort effects
Other alternative hypothesis to explain our results?
